Medical devices designed for implantation are often composed of materials that are gradually broken down by the body into products that are excreted or metabolized. Devices produced from these bioabsorbable materials tend to be more sensitive to certain physical and chemical treatments, including those necessary to sterilize medical devices. In particular, the mechanical integrity of medical devices produced from bioabsorbable polymers frequently suffers when sterilized using conventional irradiation techniques.
Ionizing radiation treatments such as gamma-irradiation, electron beam irradiation, and x-ray irradiation generate free radicals and other activated molecules that damage the biological components of contaminating bacteria, fungi, and viruses and thus ensures their inactivation. However, the constituent atoms of bioabsorbable materials are also damaged by free radicals and activated molecules, which reduces the structural integrity of the material.
Compounding the sterilization issue is the fact that steam sterilization is incompatible with thermally or hydrolytically labile polymers. Ethylene oxide, a common and widely used sterilant, often reacts with such polymers, while also requiring prolonged periods of outgassing.
In view of these issues, many new medical advances cannot be implemented because the sterilization industry is unable to provide a suitable sterilant as part of the manufacturing process. As indicated above, medical devices, such as stents, sutures, catheters and endoscopes, are fabricated from, or coated with, sensitive polymers that cannot tolerate steam, irradiation, or ethylene oxide. Moreover, plasma sterilization has been shown to be incompatible with some medical equipment and leaves toxic residues.
Issues involving sterilization exist in other areas of medical treatment as well, such as blood transfusions, blood factor replacement therapy, organ transplants and other forms of human therapy corrected or treated by intravenous, intramuscular or other forms of injection or introduction. Sterilization is also critical for the various biological materials that are prepared in media which contain various types of plasma and/or plasma derivatives or other biologic materials and which may contain harmful prions, bacteria, viruses and other biological contaminants or pathogens.
U.S. Pat. No. 5,362,442 proposes a method for sterilizing products to remove biological contaminants such as viruses, bacteria, yeasts, molds, mycoplasmas and parasites. The method proposed requires providing the product in a form that contains less than 20% solids and subsequently irradiating the product with gamma irradiation over an extended period of time. The product is irradiated for a period of not less than 10 hours. The extended irradiation time in conjunction with the low level of solids in the product is said to substantially reduce the damage to the product. The method is said to be useful in sterilizing sensitive materials such as blood and blood components.
U.S. Pat. No. 6,187,572 proposes a method for inactivating viral and/or bacterial contamination in blood cellular matter, such as erythrocytes and platelets, or protein fractions. It is proposed that the cells or protein fractions are mixed with chemical sensitizers, frozen or freeze-dried, and irradiated with, for example, UV, visible, gamma or X-ray radiation while in the solid state.
U.S. Pat. No. 6,239,048 proposes a substrate such as a woven or nonwoven fabric bound with a light-activated dye alone or in combination with additional conventional antimicrobial agents. The proposed substrate is impregnated with a light-activated non-leachable dye said to have antimicrobial and/or antiviral characteristics that can be imparted to the substrate. Upon exposure to light, the dye is reported to generate singlet oxygen that is said to kill microorganisms and viruses.
U.S. Pat. No. 6,908,591 proposes methods for sterilizing biological materials to reduce the level of one or more active biological contaminants or pathogens, such as viruses, bacteria, yeasts, molds, fungi, prions or similar agents responsible, for TSEs and/or single or multicellular parasites. The methods proposed involve the use of flavonoid/flavonol stabilizers in sterilizing biological materials with irradiation.
Despite these advances in the art, none of which address the issue of sterilizing polymeric materials, there remains a need for methods of sterilizing such materials that are effective for reducing the level of active biological contaminants or pathogens without an adverse effect on the material.